There is a need in the field of radio frequency (RF) technology to provide a duplexer (or duplex filter) for separating a transmit signal Tx and a receive signal Rx. The duplexer needs to be cheap to design and manufacture. The duplexer needs also to be of high performance, i.e. able to separate with a high degree of precision the transmit signal Tx and the receive signal Rx. The duplexer needs to withstand high RF power levels, substantially in the range of the transmit power level. With the advent of mobile communications a frequency band of the transmit signal Tx and a frequency band of the receive signal Rx are closely spaced to each other, as will be explained below. This close spacing is generally termed to be “adjacent” transmit bands and receive bands. There is usually a small separation between the transmit band and the receive band to allow the Tx and Rx parts of the duplexer an area in which to ‘roll-off’. For example, in the European 900 MHz E-GSM band, there is a 10 MHz gap between the top of the uplink (handset transmit) band and the bottom of the downlink (handset receive) band. A duplexer should provide a sharp separation of the transmit signal Tx and the receive signal Rx being relayed, i.e. transmitted and/or received at an antenna, for applications in which the transmit band and the receive band are closely-spaced.
Today's protocols for mobile communication require the sharp separation of the transmit signals Tx and the receive signals Rx, as transmit bands and receive bands are closely spaced to each other in terms of frequency. The duplexers used in mobile communications need to be adapted to handle the high transmit power levels of the transmit signal Tx and yet correctly separate the very small receive signal Rx. This is a requirement in mobile communication protocols, such as 3GPP or UMTS. The receive signal Rx typically is at a second power level. The second power level is substantially lower than the transmit power levels. Currently available duplexers that meet these requirements typically comprise large, expensive filters. Especially for applications requiring several duplexers, such as in an antenna array of a base station, the large and expensive filters may lead to high costs and a considerable size of the antenna array.
U.S. Pat. No. 5,473,295 (assigned to LK-Products OY, Finland) provides a SAW filter to a receive branch of a duplex filter. The provision of the SAW filter increases the stop band attenuation of the duplex filter. The SAW filter is configured as a notch filter. The SAW filter improves a rejection of a band pass filter in a mobile radio telephone.
Jiguo Wen, et al. disclose “Suppression of Reflection of Coefficients of Surface Acoustic Wave Filters Using Quadrature Hybrid” published in IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Volume 53, issue 10, pages 1912-1917. The Wen paper describes the use of quadrature hybrids to improve the input and output matching characteristics of a pair of identical SAW filters. Two quadrature hybrids are described in the Hashimoto publication.
It is a problem with the prior art that the out-of-band power incident upon a SAW filter can be very high (several Watts to several tens of Watts), as typically the full power of the transmitter is incident upon the SAW. Generally the SAW filters and SAW devices are limited in their capability to handle high out-of-band power levels. For example, the EPCOS B7642 SAW Duplexer is adapted to handle 1 Watt of transmit power, but only 10 Milliwatts of out of band power.
Furthermore it is difficult in the prior art to achieve a good RF power match (also termed matching) between an output of the transmitter and the SAW device and/or SAW duplexer, if transition bands of the duplexer's transmit filter and the SAW filter are close or overlapping, as is described in the LK-Products patent. Close spacing and/or even overlapping of the transition bands is desired for the duplexers in the field of mobile communication. A poor match results in poor Tx output power performance. Furthermore the poor match typically causes a poor phase response. The poor phase response results in poor transmit signal quality together with a high error-vector magnitude (EVM).